Solid electrolyte high-temperature fuel cells are essentially characterized by a solid ceramic electrolyte which is generally implemented as an oxide ceramic layer between two electrodes. A fuel cell of this kind (SOFC=Solid Oxide Fuel Cell) generally has an operating temperature of 600 to 1000° C. At this temperature, the electrochemical reaction is optimized. Low operating temperatures tend to reduce the costs of a fuel cell system.
In the power range >100 kW, primarily high-temperature fuel cell systems (SOFC) of tubular design have hitherto been operated worldwide. The associated cells are of all-ceramic design, with the different functional layers being deposited on a ceramic support. The support is located on the air side of a cell and consists of cathode material. The potential of such fuel cells has been disclosed in various publications, e.g. in “Fuel Cells Systems: Towards Commercialization” in Power Journal 2001, pp. 10-13 (publisher: Siemens AG).
An alternative to the tubular fuel cell is the planar fuel cell having a planar layer structure. Planar SOFCs are described, for example, in the publication VIK Reports “Brennstoffzellen” (“Fuel Cells”), No. 214 (November 1999), p. 49 et seq.
The high operating temperatures necessary for an all-ceramic cell are deemed acceptable, as, on the other hand, extremely low degradation values can be achieved with these systems, said values being below 0.1% over 1000 h operation which is the yardstick for evaluating fuel cells.
In the case of all-ceramic fuel cells, problems with manufacturing the cathode support may arise. Tube inhomogeneities increase the likelihood of support breakage, thereby affecting yields. In addition, all-ceramic cells require tighter manufacturing tolerances because of the large number of production steps, thereby likewise possibly limiting usability.
It has already been a matter of discussion in the industry that fuel cells operating at low or medium temperatures open up a number of new possibilities. This applies particularly to material selection for the cells and peripheral equipment. For example, low operating temperatures of approximately 600° C. permit the use of high-alloy high-grade steels and/or other metal alloys in the individual fuel cell module and associated peripherals.